2-25 DEMODULATORS IN THE SERVO SYSTEM As you know, servo systems use both ac and dc servo motors depending upon the requirements of the system. Systems that are required to move light loads at constant speed use ac motors. Systems that are required to move heavy loads with a wide speed range use dc motors. When the requirements of the system call for a dc motor or other dc devices, the ac error signal within the servo system must be converted to a dc error signal before being fed to the dc servo amplifier. The conversion is made by the circuit known as a DEMODULATOR. As with the modulator, the demodulator maintains the same relationships between its input and output signals. Just like the modulator, the demodulator's output amplitude is proportional to its input signal and its output polarity is determined by the phase of the input signal. These relationships, as in the modulator you just studied, are necessary so the "new" error signal will control the servo motor in the desired manner. Diode Demodulator One example of a servo demodulator is the DIODE DEMODULATOR, sometimes called a phase detector, shown in figure 2-18. This circuit is used in servo systems because it not only converts ac to dc, but it is also able to distinguish the phaseof the ac signal by comparing it to a reference voltage. Do not confuse this circuit with other phase detector circuits, such as those used in radar or communicationssystems. This demodulator (phase detector) distinguishes signals that are either in phase or 180º out of phase. For this reason this circuit is useful in servo systems where the ac output from the error detector (CT) is either in phase with the reference signal or 180º out of phase. Whatever type of error detector is used in the servo system, the referencevoltageto the error detector and to the demodulator must be IN PHASE with each other for the demodulator to do its job. Figure 2-18.Diode demodulator. As shown in figure 2-18, the anodes of the two diodes are supplied with the same reference voltage. With no ac error input signal applied to T2(quiescent state), both diodes will conduct equally on the positive half-cycle of the reference voltage. The voltage drops across R11 and R2are equal. This results in the two output terminals being at the same potential; therefore, the output voltage is zero for the positive half-cycle. During the negative half-cycle, a negative voltage is felt on the anodes of both diodes, both